Investigation On The Heat Transfer Enhancement Of Lithium Ion Battery Thermal Management System Based On Composite Phase Change Material

With the continuous development of electric vehicles in recent years,lithium-ion battery has attracted wide attention as energy sources for electric vehicles,and its performance and life span are restricted by the operating temperature.Therefore,battery thermal management(BTM)technology has gradually become a research hotspot.The battery thermal management system based on phase change material(PCM)has good development potential because it does not need to consume extra energy and has simple structure,but it also has the problem of low heat transfer efficiency.In view of the above problems,following researches have been done.Preparation and characterization of composite phase change materials with high thermal conductivity for BTM have been completed.Heat generation characteristics of lithium ion batteries have been measured and the dosage of composite PCM has been estimated.A new type of battery thermal management system based on high thermal conductivity and composite phase change materials has been designed and established.The heat transfer enhancement and temperature control effects have been analyzed.The main research contents and conclusions are as follows:(1)Preparation and characterization of composite phase change materials with paraffin and carbon materials for battery thermal management.Paraffin was used as phase change material,and three kinds of high thermal conductivity carbon materials(carbon nanotubes,graphene nanosheets and expanded graphite)with different dimensions of heat transfer were chosen as the high thermal conductivity materials.The mass ratio of carbon materials were 1%,2%,5%,10%of the composite PCM,the microscopic morphology,latent heat of phase transformation,phase transition temperature,thermal conductivity and other thermal properties were tested to study the effect of the type and ratio of high thermal conductivity materials on the thermophysical properties of PCM.The results show that the addition of carbon material has little effect on the melting point of paraffin,but the latent heat of phase transformation increases firstly and then decreases with the increase of carbon material.The addition of three kinds of carbon materials can increase the thermal conductivity of paraffin which increases with the increasing of carbon mass fraction.When the mass fraction of the carbon materials is 10%,the thermal conductivities of CNT/paraffin,GNP/paraffin and EG/paraffin are 0.39,2.01 and 6.39 W/(m K),respectively,which are 1.39,7.14 and22.67 times of the pure paraffin.(2)Research on heat generation characteristics of different ternary lithium ion batteries and estimation of composite phase change materials.Four types of 18650ternary Li-ion batteries from Samsung,LG,Panasonic and Sanyo were selected to conduct discharging temperature rise tests under different discharge rates(1C,2C and3C).The results show that the temperature rise and temperature difference of Sanyo NCR18650GA lithium ion is the maximum among four batteries at all discharge rates,and it has the largest rated capacity,so considering the temperature factors and the rated capacity,it is selected as the battery for assembling the battery pack.The internal resistance of the Sanyo NCR18650GA battery was tested by HPPC method.The results show that the resistance begin to decrease gradually from 52.89 m?when the SOC is higher than 80%,while the internal resistance stabilizes at 42.00 m?when the SOC is in the range of 80%to 30%When the SOC is less than 30%,the internal resistance curve increases rapidly,the maximum internal resistance of 66.98m?occurs at a SOC of 6.4%.According to the internal resistance calculation,the Sanyo battery generates heat of about 3600 J at 2C and 20?.According to the battery heat generation and the thermal properties of composite PCM,the dosage of the composite PCM for each battery must be greater than 14.4 cm~3 at 2C and 20?.(3)Research on heat transfer characteristic of lithium ion battery thermal management system based on composite phase change material.Based on the experimental results of the first two parts,two 36V10Ah battery packs were assembled.One of them was equipped with an EG/paraffin composite PCM temperature control module,and the other was assembled as a control group without temperature control module.The heat dissipation performances of this battery thermal management system were tested at four ambient temperatures of 0°C,10°C,20°C,and 30°C and at discharge rates of 1C and 2C.The experiments not only compared the temperature characteristics of different battery packs with or without phase change temperature control modules,but also studied the effects of three conditions of the dosage(overdose,appropriate,and insufficient)on the temperature of the battery pack.The results show that the average temperature of the battery increases with the increasing of the ambient temperature,because the internal resistance of the battery decreases with the increasing of the temperature,the temperature rise of the battery also decreases with the increasing of the ambient temperature.The addition of materials can effectively control the average temperature of the battery pack.After adding the temperature control module at the 1C discharge rate,the average temperature of the battery pack decrease by 32.2%,27.6%,30.9%and 28.7%when the ambient temperature is 30°C,20°C,10°C,and 0°C,respectively.The composite PCM can improve the temperature uniformity of the battery pack.When the ambient temperature is 30°C and the discharge rate is 1C,the maximum temperature difference is controlled to be less than 1.1°C,which is a decrease of 88.2%.A reasonable dosage of composite PCM is the key for its application.After the material is completely melted,its temperature control performance is obviously degraded,and the effect of the melted material on the average temperature of the battery pack is greater than the effect on the temperature uniformity.